1. Field of the Invention
The present invention pertains to the art of extrusion and, more particularly, to a die for use in an extrusion process.
2. Discussion of the Prior Art
In extruding a synthetic resin film, it is highly desirable to obtain a substantially uniform cross-section of the sheet or film. When extruding polymer films, for example, the natural tendency is for the warmest portion of the flowing material to travel more rapidly in the center of the flow channel of the die while the coolest portion of the melt travels more slowly along the outer part of the channel. This, of course, results in the formation of an undesirable non-uniform cross-section.
In one known prior art arrangement, as represented by U.S. Pat. No. 4,217,322, the die is designed to split the extruded material into two streams adjacent the entrance to the die. The two streams are then caused to flow along the edges of the die and are joined again upstream behind a dam. The dam is a narrow restriction just behind the outlet of the die. This type of arrangement has become known as a "coat hanger die".
By first splitting and then restricting the melt stream, a coat hanger type die achieves a substantially uniform flow of the melt from the die. Unfortunately, the use of the dam creates an area of high shear. Since the shear rate over the dam is inversely proportional to the square of the restriction height, as the dam becomes more restricted, the shear rate increases. Since the dam must be restricted to provide the uniform flow, a rather large shear rate is inherent in such a die arrangement. This may not present a problem in extruding various materials, however, it does create a problem in the extrusion of sheets or films which are formed from shear sensitive thermoplastic polymers such as FEP (Fluroinated Ethylene Propylene).
It has heretobefore been proposed to provide a static mixer to create a more uniform melt flow in these known coat hanger dies. These mixers are usually located within the barrel of the extruder, downstream from the end of the extruder screw. These mixers function by creating a turbulent flow in the melt stream in an attempt to prevent the gradient flow velocity described above. Unfortunately, once the molten polymer leaves the mixer, it immediately begins to form temperature gradients which again lead to non-uniform flow rates.
Therefore, there exists a need in the art for an extrusion die which can produce a sheet or film of uniform melt flow and which overcomes the problems associated with the prior art. In particular, there exists a need for a die which can be used to extrude a thermoplastic polymer material having a uniform cross-section without increasing shear rates.